The present invention belongs to a technical field covering optical components such as lenses. More particularly, the present invention relates to a durable optical component having a multilayer light reflection preventing film or the like having high adhesion force, and to a method of manufacturing the optical component.
Glass has been used as a material for optical components such as lenses. On the other hand, in recent years, optical components made of plastics such as polymethyl methacrylate (PMMA), etc. have been found in wide use because they are light in weight, low-cost and highly producible.
One of characteristics generally required for such optical components is a good light reflection preventing property (low optical reflectivity). Ordinarily, a light reflection preventing film is formed on a surface of such optical components.
A single-layer film formed by using a low-refractive-index material such as magnesium fluoride (MgF2) is generally used as a light reflection preventing film formed on optical component surfaces. As a light reflection preventing film with better light reflection preventing property, a multilayer light reflection preventing film (multiple coating light reflection preventing film) is known in which a single layer or multiple layers of a low-refractive-index material and a single layer or multiple layers of a high-refractive-index material are alternately formed.
Optical components made of plastics having a multilayer light reflection preventing film, however, have a problem that adhesion property between a substrate (i.e., the optical component made of a plastic) and the multilayer light reflection preventing film is low, that is, the durability is not sufficiently high.
Ordinarily, a multilayer light reflection preventing film is formed by a vapor-phase deposition process such as vacuum deposition.
In a vapor-phase deposition process, the temperature of the substrate rises to expand the substrate even though heating of the substrate is not performed. In particular, to form a multilayer light reflection preventing film, a process in which a plurality of layers are successively formed and film forming of a high-refractive-index material is performed by using a comparatively large amount of energy is ordinarily performed in consideration of productivity, etc. If such process is used, the substrate temperature becomes so high that the substrate expands substantially largely.
When the substrate is taken out of the film forming apparatus into atmosphere after film forming of a multilayer light reflection preventing film has been completed, it is abruptly cooled to shrink.
However, the substrate made of a plastic, particularly an acrylic material typified by PMMA and the multilayer light reflection preventing film formed on the substrate differ in thermal expansion coefficient from each other, and the thermal expansion coefficient of the multilayer light reflection preventing film formed on the substrate is smaller than that of the substrate. Therefore, the multilayer light reflection preventing film cannot follow the deformation of the substrate due to its shrinkage when the substrate is taken out of the film forming apparatus into atmosphere after the completion of film forming, resulting in damage to the multilayer light reflection preventing film and large strain in the film.
As a result, the adhesion force of the multilayer light reflection preventing film is reduced. Also, the adhesion force gradually reduces with time. Thus, the durability of the optical component having the multilayer light reflection preventing film is substantially reduced.